This invention relates to a device for detecting an angle of rotation of a rotor with respect to a stator of a brushless multi-phase d.c. motor, the rotor comprising a permanent magnet and the stator comprising a plurality of electrical windings to which, in operation, electric drive signals are applied for driving the rotor, the device having drive means for applying electric test pulses to the windings while the rotor does not rotate.
The invention also relates to a driving system comprising a multi-phase d.c. motor and a device for the detection of the angle of rotation. The invention further relates to a disk drive and a tape streamer including a driving system in accordance with the invention.
Such a device is known inter alia from U.S. Pat. No. 5,117,165. In this known device current pulses of short duration and of mutually opposite polarities are applied to each winding of the stator during standstill of the motor. Moreover, the build-up of the current in the respective winding is measured by means of a series resistor, which acts as a current-voltage converter. In this way, 2n voltages are measured in the case of a n-phase d.c. motor. It has been found that each of the measured currents depends on the position of the rotor with respect to the stator windings. With the aid of the 2n measured currents through the windings the position of the rotor with respect to the stator windings can be determined with an accuracy of 180.degree./n. Subsequently, information about the angle of rotation of the rotor with respect to the stator can be used, in a manner known per se, to start the motor in such a way that it is directly set into rotation in a predetermined direction. This is very important in the case where a multi-phase d.c. motor is used in disk drives of a certain type in order to preclude damage to the read and write heads. When multi-phase d.c. motors are used in tape streamers this is important for an optimum use of the storage capacity of a tape.
A drawback of the known device is that the build-up of currents in the windings is monitored by means of a current-voltage converter arranged in series with the respective windings. As a result of this, a comparatively large amount of power is dissipated in the current-voltage converter.